1. Field of the Invention
The present invention relates to a servo signal recording apparatus and a servo signal recording method capable of recording at least a servo signal on a magnetic tape. The present invention relates to, for example, a servo writer and a servo head unit. The present invention also relates to a magnetic tape on which a servo signal is recorded.
2. Description of Related Art
Magnetic tapes find various applications such as an audio tape, a video tape, and a computer tape. Particularly, in the field of a tape for a data backup used for a data backup of a computer, tapes with a storage capacity of hundreds of gigabytes per volume have been commercialized along with the increase in capacity of a hard disk in which a backup is to be created. In the future, the increase in capacity of a backup tape is indispensable for dealing with the further increase in capacity of a hard disk.
Along with the increase in capacity of a magnetic tape, there is a demand for high-density recording with respect to a magnetic tape. Examples of the high-density recording include a technique of recording data on a magnetic tape while shortening the recording wavelength of the data (technique of shortening a wavelength) and a technique of recording data on a magnetic tape while narrowing a track width to be recorded on the magnetic tape (technique of narrowing a track). According to the above technique of shortening a wavelength, the thickness of a magnetic layer of a magnetic tape tends to be small in order to shorten the recording wavelength of data and reduce the influence of a diamagnetic field. Furthermore, according to the technique of narrowing a track, a head for recording/reproducing data is controlled so as to follow a recording track exactly by a tracking servo (e.g., see Patent Document 1 (JP 8 (1996)-30942 A)).
However, when the thickness of a magnetic layer is reduced as described above, the output of a servo signal decreases to lower the signal/noise (S/N) ratio thereof which makes it difficult to perform exact tracking servo.
As a method for overcoming the above problem, a method for substantially doubling a servo output by demagnetizing a magnetic recording medium with a direct current (DC) before recording a servo signal on the magnetic recording medium has been proposed (e.g., see Patent Documents 1 and 2 (JP 2004-318977 A)).
According to the above configurations disclosed in Patent Documents 1 and 2, the output of a servo signal increases, whereby the S/N ratio thereof can be ensured. Furthermore, the magnetic layer can be DC-demagnetized sufficiently over the entire thickness thereof, using a wide gap erasing head with a gap length of 1 μm or more.
However, according to the configurations disclosed in Patent Documents 1 and 2, in the case where data with a short wavelength is recorded on a magnetic layer that has been DC-erased, although a data signal is recorded in an upper layer portion of the magnetic layer, a DC-erased magnetization area remains in a lower layer portion of the magnetic layer. Therefore, the S/N ratio of a data signal is lowered due to a DC erasing noise caused by a DC magnetization component, or so-called asymmetry of a reproduced waveform is caused by the change in a residual magnetization amount depending upon the magnetization direction of a data signal, which degrades an error rate.
FIGS. 7A and 7B schematically show a recorded state of a servo signal and data in a magnetic tape 9 viewed from a side. FIG. 7A schematically shows a recorded state of a servo signal in a servo track of the magnetic tape 9. FIG. 7B schematically shows a recorded state of data in a data track of the magnetic tape 9. In FIGS. 7A and 7B, an arrow X represents a magnetization direction of a magnetization area 103 of a servo signal, an arrow Y represents a magnetization direction of a DC-erased magnetization area 104, and arrows A and B are magnetization directions of respective data areas 105 and 106. The recording wavelength of a servo signal is large, so that a magnetic flux reaches a lower layer 102 during recording of a servo signal with a large recording wavelength, and a magnetization area 103 is formed up to a deepest portion of the magnetic layer 101, as shown in FIG. 7A. Thus, the DC-erased magnetization area 104 is overwritten completely. However, as shown in FIG. 7B, the magnetization areas 105 and 106 based on the recording of a data signal are formed in a surface layer portion of the magnetic layer 101 since the recording wavelength of a data signal is small. Therefore, the DC-erased magnetization area 104 remains in a lower portion of the magnetization areas 105 and 106. Thus, the above problem arises.